Protection of the population and ecosystems takes priority if there is a release of radioactivity. It is essential to identify the areas most at risk from radioactivity as soon as possible. However, before any radioactivity is released (in the pre-phase) it is not possible to make use of any radioactivity measurements obtained in the surroundings. Therefore the hazard must be estimated with the help of numerical modelling on the basis of the actual situation in the plant and the weather conditions.